Acorns are full of bitter tannins, and to make the acorns edible, they have to be leached with water. You’ve got two options here: cold or hot.
Cold-water leaching takes a little longer: the meats are put in a plastic or glass container, water is poured over the top, and the whole shebang is put in the refrigerator. Every day (or even twice a day), the water should be stirred, poured off, and replaced. Once the water starts to become less dark, try a piece. If it is still giving you a bitter aftertaste, keep going. It could take a few days or over a week to leach the acorns, depending on how bitter they are, how small the pieces are, and how often you change the water. This method is preferable if you’re making flour out of the meats because it preserves the starch.
This method is quicker, but it removes the starch from the nutmeat, so it is better if you’re going to use them as a nut or meat substitute. Pop the acorns in a sauce pan with water to cover. Heat it up to boiling and cook for 20 minutes, covered. Pour out the water and put in fresh boiling water (don’t start with cold water again as the cooling off can lock tannins in); as the previous soaking was cooking, I heated water for the next batch in a kettle. Keep cycling the water until the nut meat no longer has a bitter aftertaste.